This invention relates to fibre optic sensors and in particular but not exclusively to sensors for remotely measuring electric current.
It is known from U.S. Pat. No. 4,173,412 to utilise birefringence effects to measure transverse strain in a single mode optical fibre wound around an expanding mandrel by observing the change in polarisation state of light transmitted through the fibre. Such birefringence effects can be detected in several ways but typically light with a fixed polarisation state is input to the fibre and the output is analysed using a polarising beam splitter producing components with mutually orthogonal polarisation. The relative amplitudes of the components are processed to provide an output representative of the value of strain.
It is also known to provide transducers responsive to various physical parameters and operable to apply stress to the fibre thereby enabling the value of the parameter to be remotely sensed by observing the resulting strain in the fibre.
Sensors in which fibres are found around a mandrel have several disadvantages. Winding the coil must be done with great care since the fibre will break if wound to less than a minimum bend radius, typically several millimeters, and this also limits the extent to which such sensors may be miniaturised.
A further disadvantage is that in winding the fibre into a coil the fibre is placed in a helically sprung state in which the fibre is both transversely and longitudinally strained thereby inducing a background level of birefringence which tends to vary with age due to creep effects. The background level also tends to be unpredictable in magnitude from one winding to another such that the sensors are inherently non-reproducible.
A further disadvantage results from longitudinal strain being applied to the fibre in addition to transverse strain thereby complicating the nature of the birefringence effect produced whereas for simplicity, predictability and linearity in the observed effects it is preferable for a short linear length of fibre to be transversely strained.
It is also known from U.S. Pat. No. 4,753,507 to apply transverse strain to a fibre by means of a transducer provided with jaws which grip and compress the fibre. Such arrangements tend to damage the fibre and introduce a phase lag between transducer movement and fibre strain due to deformation of the outer jacket material of the fibre.
It is also known from GB-A-2208711 to provide a sensor in which the fibre is embedded in a body formed of an active matrix material which is inherently responsive to an external physical parameter so as to change shape and thereby apply stress to the fibre. Such sensors similarly have the disadvantages of non-reproduceability and of applying both longitudinal and transverse stresses to the fibre. Varying effects are also experienced along the length of the fibre thereby producing cummulatively a complex birefringence effect.